The overall objective of this research program is to discover the mechanisms by which the presence of an extra copy of human chromosome 21 produces the phenotype of Down syndrome (DS). The approach we are using is based on the premise that it will be possible to relate specific components of the trisomic phenotype to the increased expression of genes or sets of genes present on chromosome 21. Our work on this problem has led to the development of several animal models for DS including the full trisomy 16 (Ts16) mouse and, very recently, the new partial trisomy 16 mice, Ts1Cje and Ms1Ts65, which are trisomic from below Sod1 to Mx and from above App to above Sod1, respectively. In addition, we have studied Ts65Dn, another partial trisomy 16 mouse which is trisomic for the region App to Mx. The abnormalities of the Ts1Cje and Ts65Dn, which more faithfully reproduce the genetic imbalance that results in DS than does Ts16, are principally restricted to the nervous system and affect learning and behavior, although Ts65Dn is also male sterile. Ts65Dn, with the larger degree of imbalance, is more abnormal (in spatial learning) than is Ts108Cje. Furthermore, Ts65Dn mice display atrophy of basal forebrain cholinergic neurons (BFCN) which can be reversed by nerve growth factor, but Ts108Cje animals have normal BFCN. To determine the regions of chromosome that are responsible for the learning deficits and neuronal atrophy in the partial trisomy mouse for DS, we shall first compare in detail the behavioral differences among Ts1Cje, Ts65Dn, and Ms1Ts65. We shall then use an approach to phenotypic mapping that is subtractive in nature. It is based on the analysis of the changes in phenotype that result from decreases in the size of or the removal of specific loci from the region of trisomy. We shall analyze the effects of deleting either App or Sod1 on the phenotype of Ts65Dn. Then, starting with Ts65Dn and Ts108Cje, we shall generate two series of partial Ts16 mice with progressive radiation- induced deletions of chromosome 16. The resulting progeny will be assessed with regard to which phenotypic features of partial Ts16 disappear as extra copies of particular regions of the chromosome are no longer present. Regions so identified can then be further analyzed to identify candidate genes, and the true role of these genes in producing the phenotypic changes of partial trisomy 16 can be established by transgenic and homologous recombination techniques.